1. Field of the Invention
The present invention relates to a noise detecting circuit in a television receiver and a television receiver employing the same. More specifically, the present invention relates to an improvement in detecting with accuracy a noise component in a detected composite video television signal and an improved television receiver employing a control responsive to the noise detected output.
2. Description of the Prior Art
Generally, in reception by a television receiver of a television broadcasting signal of a small electric field intensity or of a weak electric field intensity, a high frequency amplifying circuit and an intermediate frequency amplifying circuit having an automatic gain control function operate with the most increased gain. The magnitude of a noise component such as a white noise included in a detected composite video signal obtained from a video detector is accordingly increased. When such a noise component becomes larger in the composite video signal, horizontal synchronization is liable to be interfered with and hence a contour line in the vertical direction or a circle pattern on the screen is flared or a portion of an image on the screen is moved in the horizontal direction. Thus, horizontal jitter is caused on the occasion of weak electric field intensity. Such phenomenon is caused because the response sensitivity of a horizontal synchronization automatic frequency control circuit is rather good and the control signal thereof fluctuates in response to the noise component, whereby the oscillation frequency of the horizontal oscillating circuit accordingly fluctuates in response to the fluctuation of the control signal. Therefore, one approach to eliminate such inconvenience is to decrease the fluctuation of the oscillation frequency of the horizontal oscillating circuit by decreasing the sensitivity of the horizontal synchronization automatic frequency control circuit in accordance with the magnitude of a noise component included in the composite video signal.
On the other hand, it is necessary to maintain synchronization of the horizontal output signal with the horizontal synchronizing signal by changing the oscillation frequency of the horizontal oscillating circuit when the horizontal output signal from the horizontal output circuit becomes out of synchronization with the horizontal synchronizing signal. To that end, it is necessary to make fast the phase control by increasing the sensitivity of the horizontal synchronization automatic frequency control circuit in the case where the horizontal output signal becomes out of synchronization with the horizontal synchronizing signal. However, whereas it is possible to decrease occurrence of the horizontal jitter when the noise component in the composite video signal is increased by decreasing the sensitivity of the horizontal synchronizing automatic frequency control circuit, it becomes difficult to make the horizontal output signal follow the horizontal synchronizing signal when the horizontal output signal becomes out of synchronization with the horizontal synchronizing signal. Conversely, assuming that the sensitivity of the horizontal synchronizing automatic frequency control circuit is increased, it is possible to make the oscillation frequency of the horizontal oscillating circuit follow that of the horizontal synchronizing signal more quickly when the electric field intensity is large but it becomes difficult to decrease a horizontal jitter when the noise component in the composite video signal is increased on the occasion of a weak electric field intensity. Thus, it is difficult to control the response sensitivity of the horizontal synchronization automatic frequency control circuit so that a horizontal jitter may be decreased even when a noise component in the composite video signal is increased on the occasion of a weak electric field intensity and the horizontal output signal may be synchronized with a horizontal synchronizing signal on the occasion of a strong electric field intensity.
One example of a horizontal synchronization automatic frequency control circuit adapted for providing a good picture quality on the occasion of either a strong electric field intensity or a weak electric field intensity is disclosed in the U.S. Pat. No. 3,334,182, issued Aug. 1, 1967 to Ernst Legler. The referenced U.S. patent comprises a dual automatic frequency control circuit including the first line synchronizing circuit and the second line synchronizing circuit. However, the above described dual automatic frequency control circuit of the referenced U.S. patent involves a shortcoming in that the circuit configuration is complicated.
Therefore, it is desired that a television receiver of a relatively simple structure be provided that is capable of reproducing on the screen a picture of a good quality by controlling the response sensitivity of a horizontal synchronization automatic frequency control circuit on the occasion of either a strong electric field intensity or a weak electric field intensity and even in the case where the horizontal output signal becomes out of synchronization with the horizontal synchronizing signal. To that end, it is necessary to detect the magnitude of a noise component included in a detected composite video signal. One possible approach for detecting a noise component in a detected composite video signal is to utilize an automatic gain control voltage. However, as well known, an automatic gain control voltage is rather representative of a combination of a noise component and a signal and for this reason does not accurately represent a noise component itself in a detected composite video signal even though an electric field intensity is accurately detected by way of an automatic gain control voltage.